In the field of wheel anti-skid brake control systems for motor vehicles, the so-called individual wheel control or regulation is considered the most effective, but it is also the most involved and expensive. In the case of individual wheel regulation, the anti-skid brake control system is designed in such a way that a velocity sensor is associated with each vehicular wheel. Thus, the dynamic or rotational behavior of each wheel is separately scanned and is employed to control the magnetic solenoid valve and associated brake cylinder over its own regulating channel. In such anti-skid braking system, each wheel is separately regulated so that optimum slip results, and thus the vehicle exhibits a minimum stopping distance while maintaining its steering capability.
Furthermore, it is common practice to provide a safety switching arrangement in some of the previous anti-skid brake control systems. The safety switch senses and monitors the signals which are supplied by the sensor or those that are used to control the magnetic solenoid valve for being present for an undue period of time. Such an over-extended duration generally means or signifies that the signals are lasting too long. When such an overtime signal period is determined, the regulating portion of the vehicle braking system is turned off or shut down by the safety switch, and thus the system reverts to normal braking operation without any regulation.
Now if the anti-skid system is functioning to provide regulating braking of the vehicle, and the safety switch suddenly turns off the regulation due to the determination of a faulty condition, then the magnetic solenoid valves are placed in their open positions. Thus, the driver of the vehicle assumes the braking operation, and the brake pressure is completely controlled by the foot pedal control valve. In many cases, the driver exerts excessive pressure on the foot pedal so all the wheels of the vehicle are suddenly locked-up. Under this condition, the wheels lose lateral stability with the roadway, and the vehicle can no longer be properly steered since it tends to skid sideways.
Furthermore, in conventional anti-skid brake control systems, there is also a possibility that a malfunction or fault can occur in the safety switch itself. When this happens, the overall security of the anti-skid system may be entirely lost since a simultaneous failure in the regulation apparatus may go unnoticed and undetected.
Additionally, it is well known as shown and described in U.S. Pat. No. 4,088,376 to develop a vehicle reference speed from the velocities of at least two wheels and then to compare them with the actual wheel velocity or ground speed. In practice, at least one nondriven wheel, for example, a front wheel, is utilized to develop the minimum reference velocity since the rear wheels which are very sluggish due to their connection to the transmission and, in turn, the engine, can be easily stabilized and regulated even on a road that has a very low coefficient of friction.
However, a problem can occur when a common vehicle reference velocity is employed to regulate the braking of all of the wheels since an erroneous reference velocity may actually result in the loss of braking on several of the wheels of the vehicle.